The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
The economic viability of geothermal energy production depends on finding a subsurface geothermal resource with high enough temperature, which will yield high enough flow rates per well to justify exploration and well development costs. Insufficient working fluid and pressure depletion can limit well flow rates and increase the cost of powering the working-fluid recirculation system. Depending on geothermal resource temperature and geothermal reservoir permeability, the parasitic cost of driving the working-fluid recirculation system can be high, sometimes consuming more than half of the gross geothermal power output. A key goal in optimizing geothermal energy production is thus to minimize the parasitic cost of powering the working-fluid recirculation system. Because well costs constitute a major portion of capital costs, another key goal is to increase flow rates on a per well basis.
Besides geothermal resource temperature, two key factors affecting the parasitic cost for driving working-fluid recirculation in liquid-dominated, hydrothermal geothermal power systems are the permeability of the geothermal reservoir formation and whether artesian pressure exists in that reservoir to help drive flow up production wells. Without sufficient artesian pressure, extracting heat requires that formation brine be lifted up production wells, such as with submersible pumps, which can consume a significant portion of the electricity generated by the power plant. Sedimentary formations are attractive candidates for geothermal power production because they have the advantages of higher reservoir permeability and much larger areal extent, compared to hydrothermal systems in crystalline rock formations where conventional geothermal power systems are usually deployed. See, for example, Buscheck, T. A., Chen, M., Lu, C., Sun, Y., Hao, Y., Celia, M. A., Elliot, T. R., Choi, H., and Bielicki, J. M., “Analysis of Operational Strategies for Utilizing CO2 for Geothermal Energy Production,” Proceedings of the 38th Workshop on Geothermal Reservoir Engineering, Stanford University, Palo Alto, Calif., 11-13 Feb. 2013. High permeability and large areal extent are conducive to higher per well flow rates for both injection and production wells. Recently, carbon dioxide (CO2) has been considered as a working fluid because its advantageous properties reduce the parasitic cost of working-fluid recirculation. Moreover, the low viscosity of CO2, compared to formation brine, and the thermosiphon effect promote higher per well flow rates for both injection and production wells. See, for example, Brown, D. W., 2000, “A Hot Dry Rock Geothermal Energy Concept Using Supercritical CO2 Instead of Water”, Proceedings of the 25th Workshop on Geothermal Reservoir Engineering, Stanford University, 233-238; Pruess, K., 2006, “Enhanced Geothermal Systems (EGS) Using CO2 As Working Fluid—a Novel Approach for Generating Renewable Energy With Simultaneous Sequestration of Carbon”, Geothermics, 35, 351-367; Saar, M. O., Randolph, J. B., and Kuehn, T. H., 2010, “Carbon Dioxide-based Geothermal Energy Generation Systems and Methods Related Thereto”; US Patent Application 20120001429; Randolph, J. B., and Saar, M. O., 2011, “Combining Geothermal Energy Capture With Geologic Carbon Dioxide Sequestration”, Geophysical Research Letters, 38. It is worth noting that nitrogen (N2) also has advantageous properties that reduce the parasitic cost of powering the working-fluid recirculation system. See, for example, Buscheck, T. A., Chen, M., Hao, Y., Bielicki, J. M., Randolph, J. B., Sun, Y., and Choi, H., “Multi-Fluid Geothermal Energy Production and Storage in Stratigraphic Reservoirs”, Proceedings of the Geothermal Resources Council 37th Annual Meeting, 2013.
With the growing penetration of renewable energy sources in electrical grids, there is currently a strong need for systems and methods which can help to increase the reliability and dispatchability of these energy sources.